A light-sensitive coating (eg on a glass master). After exposing to light (eg using a laser) it can be developed so that the exposed parts are removed.. Photoresist is a chemical sensitive to light within a specific range of wavelengths. When exposed to light within that range, the photoresist undergoes a chemical change.
During optical disc mastering, a glass substrate coated in photoresist is selectively exposed by the mastering laser. After exposure, the glass is submerged in a developing solution. Both positive and negative photoresists are available, the difference being that the developing solution attacks the exposed areas of a positive photoresist and the unexposed areas of a negative photoresist.
The photoresist which remains on the substrate after developing is a physical template of the structure of the disc.